The invention relates to the reduction of stress between contiguous layers of different semiconductor materials and, more particularly, to double heterostructure junction lasers with reduced current density thresholds for lasing.
In the summer of 1970 I. Hayashi and M. B. Panish reported successful c.w. operation at room temperature of a semiconductor p-n junction laser now known as the double heterostructure (DH) laser (Applied Physics Letters, Vol. 17, No. 3, pages 109-111, Aug. 1, 1970). This achievement was the result of intensive efforts both in physics, which led to the design of the DH laser, and in chemistry, which led to improved liquid phase epitaxy (LPE) techniques for realizing the DH laser.
Briefly, the DH laser comprises a narrow bandgap active region, which may be either n-type, p-type or contain a p-n junction in which case it has both conductivity types. The active region is sandwiched between relatively wider bandgap, opposite-conductivity-type layers which form two heterojunctions, one at each interface with the active region. These heterojunctions, as is now wellknown, serve to confine injected carriers as well as stimulated radiation to the active region. Consequently, it was early recognized by Hayashi and Panish that the heterojunctions should have as few defects as possible because such defects could act as nonradiative recombination centers which would tend to reduce efficiency and increase lasing thresholds. They, therefore, fabricated their DH lasers by LPE from the GaAsAlAs semiconductor system; i.e., early forms of the DH lasers comprised an n-GaAs substrate on which were grown the following layers: EQU n-Al.sub.x Ga.sub.1.sub.-x As, p-GaAs (the active region) and p-Al.sub.x Ga.sub.1.sub.-x As.
Because GaAs and AlAs are nearly lattice matched at the growth temperature (about 800.degree.C), GaAs and AlGaAs were even better lattice matched, so that particularly good heterojunctions were formed during LPE fabrication.
When operated c.w. at room temperature, however, these early forms of the DH laser typically had relatively short lifetimes ranging from only a few minutes to tens of hours. Consequently, systematic studies of DH lasers were undertaken in order to identify degradation mechanisms and to develop solutions to the short lifetime problem.